PostgreSQL's DISTINCT ON
clause simplifies retrieving the first row for each unique value within a set. However, a common pitfall arises when the DISTINCT ON
expression doesn't align with the initial ORDER BY
expression.
DISTINCT ON
Expression Mismatch ErrorThis error frequently occurs:
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Resulting in:
1 |
|
DISTINCT ON
and ORDER BY
PostgreSQL's documentation mandates that the DISTINCT ON
expression(s) must mirror the leftmost ORDER BY
expression(s). The solution is straightforward: reorder your ORDER BY
clause:
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address_id
OrderingIf you need to avoid ordering by address_id
, consider these alternatives:
Method 1: The "Greatest N per group" Approach
This method efficiently finds the most recent purchase for each address_id
:
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Method 2: Nested Query for PostGreSQL
This approach uses a subquery to achieve the desired result while maintaining the purchased_at
ordering:
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These alternatives provide flexibility when you don't want to prioritize address_id
in the final output ordering. They effectively address the "most recent purchase per address" problem without directly violating the DISTINCT ON
constraint.
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